Editing IPCC:AR6/WGI/Chapter-9 (section)

== 9.7 Final Remarks ==

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The process-based assessment of observed and projected change in the ocean, cryosphere and sea level undertaken here reveals advances and gaps in reconstructions, observations, models and process understanding. Revisiting the updated assessments since AR5 and SROCC helps to gauge the robustness of understanding and quantitative assessments. The CMIP6 family of models builds on the experience of the CMIP5 models, and the projections of ISMIP6, LARMIP-2 and GlacierMIP strengthen understanding. Taken together with emulators of these simulations (Box 9.3) and transparent statistical approaches ( [[#9.6.3|Section 9.6.3]] ), this chapter provides projections that are consistent with the assessment of equilibrium climate sensitivity in this Report and that have improved estimates of uncertainty.

The largest uncertainties in future sea level and cryosphere change are related to the Greenland and Antarctic ice sheets (Sections 9.4.1.3, 9.4.1.4, 9.4.2.5 and 9.4.2.6). While the ISMIP6 and LARMIP-2 protocols provide simulations permitting uncertainty estimation and probabilistic inferences, remaining ''deep uncertainty'' relates to ice-sheet processes and the atmospheric and oceanic conditions simulated by CMIP models in polar regions (Sections 9.4.2.3 and 9.4.2.4). ISMIP6 and LARMIP-2 have not been simulated beyond 2100, which greatly reduces the amount and variety of state-of-the-art projections available to make ice-sheet and sea level projections beyond 2150. After 2150, ''limited agreement'' causes us to consider all projections as ''low confidence'' . Critically, the uncertainty in ice-sheet projections is the leading uncertainty in projections of future global sea level for the second half of this century and beyond ( [[#9.6.3|Section 9.6.3]] ).

Glacier inventory and projection uncertainty has been a significant source of past sea level budget uncertainty and remains a dominant uncertainty until mid-century. Emissions scenario becomes the largest source of glacier change uncertainty by 2100, just as the relative importance of glacier loss is projected to decrease ( [[#9.5.1|Section 9.5.1]] ).

New high-resolution climate models show that sea surface temperature, overturning circulation, ocean heat content change and sea ice cover are considerably improved in most models when compared to the coarser resolution models. Change in the Southern Ocean and adjacent shelves ( [[#9.2.3.2|Section 9.2.3.2]] ) is intimately linked to the future of the Antarctic Ice Sheet ( [[#9.4.2.3|Section 9.4.2.3]] ), and projection of the Southern Ocean depends on oceanic and atmospheric drivers affecting heat (and carbon) uptake and sea ice. However, resolution remains a factor, as most CMIP6 models are far from resolutions that directly represent coastal and regional shallow-water processes, such as those beneath Antarctic ice shelves, in Greenland fjords and the eddying convection found by the Overturning in the Subpolar North Atlantic Program.

Processes that change on long time scales – particularly Atlantic Meridional Overturning Circulation, ocean heat content, and ice sheets – require additional projections beyond the CMIP scenarios to explore longer-term commitment, post-forcing recovery measured in centuries rather than years or decades, and potential tipping points and thresholds. Only a few new studies focused on longer time scales, and none based on CMIP6 models.

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